1. Field of the Invention
This invention is in the field of burning fine grained materials such as employed in the manufacture of cement clinker wherein improved efficiency and improved quality are achieved by passing at least a portion of the exhaust gases through a steam generator to remove noxious substances and to recover the thermal energy in the gases.
2. Description of the Prior Art
Raw cement meals used as starting materials for the manufacture of cement often include undesirable substances such as alkalies, chlorides and sulfur. The undesirable material such as sulfur and chlorine are often found in fuels employed for the thermal treatment of the raw cement meals, particularly when the fuels are of low grade. The alkali problem is of particular significance in cement technology from two points of view. On the one hand, it has been recognized that even slight amounts of alkali oxides in the cement lead to setting problems in the cement. Secondly, particular difficulties arise in modern cement burning systems which utilize suspension type drying processes due to the high alkali and sulfur contents in the raw cement meal and/or in the fuel. These difficulties arise not only because of the altered flow behavior of the raw meal in the heat exchanger, but the noxious substances present also may cause incrustations and caking phenomena in the gas-carrying conduits which make it impossible to provide a careful regulation or control of the burning system. This occurs because the alkali compounds in the sulfur are also included in the substances which are volatilized in the sintering kiln such as the conventional rotary tubular kiln of the burning system. These materials become sticky when they condense out of the exhaust gas of the rotary tubular kiln and lead to caking in the gas-carrying conduits. Moreover, these noxious compounds proceed with the exhaust gas of the rotary tubular kiln into the raw meal preheating system where they precipitate onto the comparatively cooler raw meal and are returned into the rotary kiln in circulation with the preheated raw meal, whereby a highly concentrated circulation of noxious substances can arise in the burning process.
It is known, for example, from DE-OS No. 30 16 642 to withdrawn the hot exhaust gas from the rotary kiln or at least a portion thereof from the burning process by means of a bypass conduit, to cool it down to about 450.degree. C. by the addition of cool air, and subsequently cleanse it of the noxious substances and thereafter discard it. Considerable heat content, however, of the thermally valuable exhaust gas from the rotary tubular kiln is thus lost for the burning process so that the entire burning process can become uneconomical. Particularly in the case of extremely large cement clinker burning systems, it can no longer be justified in terms of heat economy to discard too much rotary kiln exhaust gas which contains noxious substances without exploiting its heat content, since the cement manufacturing costs become too high due to current and anticipated energy prices. A utilization of the bypass gas discarded at about 450.degree. C. and largely freed of its noxious substances would indeed be possible but it is no longer very economical due to thermodynamic reasons. Morever, with the known bypass gas treatment, the volume of exhaust gas to be dedusted is increased up to five-fold due to the admixture of fresh air in the gas cooler, whereby the gas cleaning devices must be of large dimensions.